The constant progress in semiconductor technology demands for the fabrication of ever smaller devices. This development has to be accompanied by concurrent improvement in metrology capabilities, in order to monitor and control the fabrication process.
Over the last few decades, optical critical dimension (OCD) metrology has taken a pivotal role in semiconductor manufacturing process, due to its extreme sensitivity, accuracy, flexibility and speed. In order to provide adequate improvement of the metrology capabilities, OCD tools have gone through extensive improvement and refinement, and can provide today extremely accurate broadband spectral measurements and extremely high throughput.
In addition to the process of improving the basic tool characteristics, another venue by which OCD performance can be improved is through diversifying the measured information. Commonly measured optical properties are the reflectivity for different incidence angles, azimuths, polarizations and wavelengths. In addition, the relative phase between reflected TE and TM polarization components can be accessed through (e.g.) ellipsometric measurements.
Traditional optical wafer metrology tools rely on spectral reflectometry and/or ellipsometry. However, the electromagnetic field scattered from a sample also contains spectral phase information, which may be highly beneficial in extracting further or more accurate information from the measurements. This quantity describes the relative phase between the incident and reflected electromagnetic waves. Typically, this phase has different values for different wavelengths, incident angles\azimuths and polarizations.
Since accessing the phase directly is not possible at optical frequencies, one has to use interference effects, usually observed with an interferometer, and recover the encoded phase information from the interference effects. Most interferometers consist of a split optical path that is recombined to form interference fringes. One arm of the path is kept as a reference, and the other interacts with the sample. The interference signal from these two components is then used to extract the spectral phase, since the phase change incurred by the sample causes a change in the fringe pattern of the recombined beams that may be measured.
In many cases it is advantageous to perform optical wafer metrology measurements at oblique angles to the sample. Depending on the wafer stack and pattern type, measurements at oblique angles may provide additional information that improves the quality of information extracted from the measurements. This reasoning also carries over to interferometric measurements. As opposed to Ellipsometric phase, which provides relative phase on s and p polarizations, the Interferometric measurement provides phase relative to a well characterized reference mirror, and also provides absolute phase value or a change of phase of light returned from a sample relative to the original phase of incident light.
In conventional metrology tools, an oblique measurement scheme is usually implemented in a brightfield configuration with separate illumination and collection objectives set opposite each other so that the collection objective is aligned to receive the specular reflection from the sample. This is schematically illustrated in FIG. 1.
General Description
There is a need in the art for optimizing both spectral and interferometric measurements, thereby increasing the amount of information about a sample under measurements.
The present invention provides a novel optical system which utilizes oblique measurement scheme for a spectral interferometer. The optical system of the invention may be used in metrology measurements on patterned samples (e.g. semiconductor wafers), as well as in (phase sensitive) microscopes utilizing an oblique configuration.
The measurement system of the invention includes an optical system which is configured to define at least an oblique channel system (optical scheme). As noted above, in such optical scheme, incident light is directed from a broadband light source along an oblique illumination channel onto a measurement plane (where a sample is located), and broadband light specularly reflected from the sample is directed along a collection channel to a detection device (including a spectrometer). This optical system includes an interferometric unit typically including a beam splitting/combining device and a reference reflector device. The beam splitting/combining device splits incident light into sample and reference light beams propagating in sample and reference paths, and combines reflected reference and sample beams into the collection channel to enable creation of a spectral interference pattern on a detection plane.
The relative location of the measurement plane, splitting/combining surface(s) and reference reflecting surface (as well as other reflecting surface as used in some embodiments), and angular orientation of such surfaces with respect to an incidence plane and a normal plane are properly selected/controllably tunable to provide a desired relation between reference and sample arms in the interferometer, i.e. provide desired optical path difference between the reference and sample arms. The incidence plane is perpendicular to the measurement plane, and in the specular-reflection configuration includes the illumination and collection channels. The normal plane is a plane perpendicular to the measurement plane and to the incidence plane. The reference arm is formed by the optical path of the reference beam between the splitting and combining locations, and the sample arm is formed by the optical path of the sample beam between the splitting and combining locations.
In some embodiments, the beam splitting/combining device is configured to define one or more beam splitting surfaces and one or more beam combining surfaces located in different planes which are spaced-apart along an axis normal to the measurement plane and are substantially parallel to one another and to the measurement plane and reference reflective surface.
In some embodiments, the beam splitting/combining device may be configured to define at least one beam splitting surface and at least one beam combining surface located in the same plane substantially parallel to the measurement plane and to the reference reflective surface. In some embodiments, the interferometric unit is configured such that the reference arm forms a mirror image of the sample arm with respect to the plane containing the beam splitting and combining surfaces.
In some other embodiments, the beam splitting/combining device is configured to define at least one beam splitting surface and at least one beam combining surface located in spaced-apart relationship and oriented substantially symmetrically with respect to the normal plane.
The interferometric unit may be configured such that the sample and reference paths form a mirror image of the reflected sample and reference paths with respect to the normal plane.
In some embodiments, the beam splitter/combiner device comprises at least one pellicle structure comprising at least one partially-reflective region. In some examples, the partially-reflective region may be located on and extending along either one of its opposite surfaces. In some other examples of these embodiments, the pellicle structure comprises a first partially-reflective region located on a first of its opposite surfaces and being aligned with a substantially transmitting region on a second of the opposite surfaces, and a second partially-reflective region located on the second opposite surface and aligned with a substantially transmitting region in the first surface. In some other examples of these embodiments, the partially reflective region divides the pellicle structure into two substantially identical transmitting pellicles at opposite sides of said region. In yet further possible examples of these embodiments, the beam splitter/combiner device comprises first and second pellicles located in, respectively, the oblique illumination and collection channels, and each pellicle comprises the at least one partially-reflective region.
In some embodiments, the reference reflector device comprises at least one reflective surface, which may be oriented substantially parallel to the measurement plane or with a certain tilt configuration. The reference reflector device may be configured as pellicle structure, comprising the reflective surface on either one of its opposite sides.
In some embodiments, the system is configured to be selectively shiftable/modifiable between the oblique spectral interferometric mode and oblique spectral reflectometric mode, or concurrently/independently operable in both of these modes. Alternatively or additionally, the system may be configured to be selectively shiftable between the oblique spectral interferometric mode and normal-mode reflectometric measurements, or concurrently/independently operable in both of these modes. If the addition of normal channel configuration, for either one or both of spectral interferometric and reflectometric modes, the system may include additional light source and/or additional detector associated with the normal channel scheme.
In some embodiments, the system also includes one or more light propagation affecting elements controllably operable to selectively shift the system operation between different modes. This may for example be a blocking mechanism (shutter) controllably operable to selectively block the reference path of the interferometer, thus causing the system to operate in the reflectometric mode.
In some embodiments, the same beam splitter/combiner device is used for implementing both oblique—and normal-channel spectral interferometry. Also, in some embodiments, the same reference reflector device is used in both oblique—and normal-channel spectral interferometric measurements In some other embodiments, the normal channel optical system utilizes a separate beam splitter device, and possibly also a separate reference reflector.
In some embodiments, the optical system further comprises one or more folding reflecting surfaces located in at least one of the reference and reflected reference paths. This enables selective operation of the system in either one of the oblique spectral interferometric mode and oblique spectral reflectometric mode.
As indicated above, the system includes one or more driving mechanisms for controllably displacement of the system elements. The displacements include one or more of the following: displacing the measurement plane along the normal axis; and displacing at least one of reflecting and partially reflecting surfaces of the optical system. The controllable displacement(s) enables at least one of the following: adjusting an optical path difference between the reference and sample arms in the interferometric unit; and adjusting a shift of the system operation from a spectral oblique interferometric mode to one or more of spectral oblique reflectometric, spectral normal interferometric, and spectral normal reflectometric modes.
The driving mechanism associated with either one of the reflecting and partially reflecting surfaces of the optical system is configured for executing at least one of the following displacements of the respective surface: displacement along the normal axis; angular displacement/tilt in the incidence plane with respect to the measurement plane; and angular displacement/tilt with respect to the incidence plane.
The system also includes or is connectable to (via wires or wireless signal transmission of any known suitable type) a control unit. The control unit is configured and operable to receive and process measured data (from the detection device(s)), and to operate one or more of the driving mechanisms.
Thus, according to one broad aspect of the invention, there is provided a measurement system for use in metrology measurements on patterned samples, the system comprising: at least one light source device configured to generate broadband light, at least one detection device configured for providing spectral information of detected light, and an optical system comprising at least an oblique channel system configured to direct incident light generated by said at least one light source along an oblique illumination channel onto a measurement plane, on which a sample is to be located, and to direct broadband light specularly reflected from the sample along a collection channel to said at least one detection device, wherein said optical system comprises an interferometric unit comprising a beam splitting/combining device and a reference reflector device, the beam splitting/combining device being accommodated in said illumination and collection channels and configured to divide light propagating in the illumination channel into sample and reference light beams propagating in sample and reference paths, and combine reflected reference and sample paths into the collection channel to thereby create a spectral interference pattern on a detection plane defined by said at least one detection device.